The present invention relates to a method of manufacturing magnetic recording media, such as magnetic disks and magnetic tapes.
Magnetic recording technology has a long history and is based on established techniques. The development of an information-based society in recent years has accelerated the development of magnetic recording media of higher density with improved properties. Such technology has become more and more important.
In particular, a thinner recording layer has been used for higher recording density in the field of magnetic disks (e.g., hard disks and floppy disks) used as peripheral storage devices. The Age of Thin Film Media using a magnetic thin film has begun. Important challenges are how to ensure the reliability of data stored thereon with a higher recording density and how to make such media suitable for mass-production at a low manufacturing cost.
One of the thin film recording layers meeting such requirements consists of a magnetic layer of a cobalt-platinum (CoPt) based alloy having a high coercive force, which is divided into two magnetic films by a nonmagnetic layer of chromium (Cr) for the reduction of noise during reproduction. More specifically, the resultant medium is formed from a magnetic CoPt film, a nonmagnetic Cr film, and another magnetic CoPt film in this order. It is also known that an underlying layer of Cr provides a favorable crystalline structure for the magnetic CoPt film when placed under the thin film recording layer (as disclosed in Japanese Patent Laid-Open No. 2-210614).
However, the above-mentioned magnetic recording medium with the underlying Cr layer, magnetic CoPt film, nonmagnetic Cr film, and magnetic CoPt film placed in this order exhibits insufficient or unsatisfactory properties for a recording medium, such as coercive force, reproduction output voltage of a magnetic head (product of residual magnetization and film thickness of the magnetic layer), and signal-to-noise ratio when it is manufactured with an in-line sputtering apparatus.
In addition, it has been found that the magnetic recording medium having the above-mentioned structure which is called a double layer structure, shows coercive force in comparison with a single magnetic layer and shows poor overwriting properties.
On the other hand, it is known that a higher coercive force can be achieved by using chromium as the underlayer of the magnetic layer, such as CoNiCr alloy or CoCrTa alloy (see, for example, IEEE Transactions on Magnetic, Vol. MAG-3, No. 3 (1967), pages 205-207).
However, the magnetic CoPt alloy layer causes poor orientation of C-axis of h.c.p. structure when only chromium (Cr) is used as an underlayer. This is because the lattice constant of the magnetic CoPt-based alloy layer is larger than the crystalline lattice constant of the conventional magnetic layer of the CoNiCr alloy or the CoCrTa alloy. As a result, the magnetic CoPt-based layer is not completely matched in atomic alignment with the underlying layer of the single component of Cr. This badly affects the orientation of the C-axis.
In order to overcome this problem, U.S. Pat. No. 4,652,499 proposes to add a second or different metal to the Cr underlayer to improve the lattice constant. The orientation along the C-axis can be improved for the magnetic layer in a film interface or boundary by means of changing the lattice constant of the alloy underlayer by addition of the different metal to the Cr alloy.
The present inventors have found, as a result of detailed consideration, that the resultant recording medium has a significantly larger noise as a result of the addition of the different metal to the underlying Cr layer.